Lead alloys with a relatively high antimony content (currently approximately 4 to 10 wt-% Sb) are used as casting material for the electrodes of lead batteries for cyclical stresses Lead/antimony alloys have advantages both during the manufacturing process of the electrode frames (improvement of the flow characteristics of the molten metal in the moulds, greater hardness of the cast electrode frame) and during use of the battery; particularly in the case of cyclical loads, a good contact between terminal and active material is constantly ensured at the positive electrode in addition to mechanical stability, so that a premature drop in capacity does not occur ("antimony-free" effect).
However, antimony-containing positive electrodes have the disadvantage that antimony is dissolved ionically, migrates through the separator and, because it is nobler than lead, is deposited on the negative electrode. This process is described as antimony poisoning. Through a reduction of the overvoltage for hydrogen, it leads to increased water consumption and thus requires more maintenance. Attempts have already been made to completely or partially replace the antimony in the lead alloy with other alloy components, which, however, has not led to satisfactory results.
For the electrical separation of the positive from the negative electrode plates in storage batteries, porous separators are used which are arranged between the plates. In deep cycle batteries the most varied materials have already been tried and used as separators, but until now no system has been able to meet all requirements with the same degree of satisfaction. The problem of antimony poisoning in particular has not yet been satisfactorily solved; reducing or delaying the thus-caused effects could considerably lengthen the life of batteries
Battery separators made from rubber are known In German patent 414 975 a rubber membrane made from latex is described. Like all natural substances (previously, even wooden separators were used for example), this system has only insufficient chemical stability vis-a-vis sulphuric acid and oxidative attacks, so that the necessary battery life is not reached.
A more recent version of a flexible rubber separator is described in U.S. Pat. No. 4 213 815. Here a mixture of natural rubber and copolymers, pore-formers and auxiliary agents is laminated onto a macroporous fleece, and the polymer mixture is then partially crosslinked by electron beams. In addition to the only low chemical stability vis-a-vis sulphuric acid and oxidative attack, this separator is over-flexible, so that it cannot give the necessary support to the negative electrode mass to prevent overexpansion in cyclically charged cells.
Improved chemical stability and more efficient support of the negative electrode mass is achieved by using porous hard-rubber separators. Such separators are described for example in German published applications 1 496 343 and 1 596 296. For process-related reasons, hard-rubber separators can only be manufactured with comparatively low porosity and thus display high electrical resistance for the necessary ion stream in the battery. Moreover, as a result of the vulcanization, the hard-rubber structure filled with silica becomes brittle, so that incorporation into the cell proves very difficult.
To improve this situation, it has been proposed to laminate hard-rubber separators to a woven or non-woven material. German published application 29 24 239 describes for example the lamination of a filled, vulcanized rubber layer to a polyester fleece, thus producing in turn separators with comparatively high electrical resistance or, if sheets of very thin thickness are produced, giving rise to cracks due to the brittleness of the hard-rubber and a lack of support effect
A similar embodiment is known from German published application 30 05 297 in which the microporous separator is provided with a glass-wool fleece on the side facing the negative plate. In addition to an increase in the electrical resistance, the danger exists with such separators that charging gases will accumulate in the glass-wool fleece and thus trigger off a series of undesired secondary reactions. This problem is even more pronounced if the entire space between the electrodes is filled, as is proposed for example in German patent 944 440. Here a glass-fiber fleece is processed with a paste made of mineral substances and rubber as binding agent to produce a separator.
Nowadays, separators based on thermoplastic or thermosetting plastics are predominantly used as separators for lead/lead dioxide batteries with cyclical stress. Especially widely used is a separator which consists of high-molecular-weight polyethylene containing as pore formers silica and a mineral oil, cf. German patent 1 496 123. A likewise extensively used microporous separator made of a thermoset resin is proposed in German patent 1 596 109. Moreover, there are other separators based on polyvinyl chloride or mixtures of other polymers and also with and without pore forming fillers. For applications with average cyclical loads, separators based on glass-fibers are also used. All these separators made of synthetic plastics or glass-fibers have the characteristic that they have little or no effect on the chemical reaction sequences in an battery -- also not an advantageous one, e.g. delaying the antimony poisoning. On the other hand their chemical stability and their low electrical resistance are very advantageous.
It has been shown that the processes connected to antimony poisoning can be influenced by separators; however, the mechanism of their influence is unknown and the wooden or hard-rubber separators considered in this context display other serious disadvantages, as previously mentioned.
Previous theories and proposals predominantly assume a capture mechanism on the basis of a chemical reaction with the antimony Thus, sulphur (Japanese published application 60-250 566) or organic sulphur compounds including hard-rubber (German published application 31 11 473), resins with aminophosphonic groups (French patent 2 440 085), tannic acid, sodium alizarin sulphonate, salicylic acid, p-cresol, resorcinol, pyrogallol, hydroquinone, catechol et al. (Japanese published application 55-80 267), ethylene diamine tetraacetic acid (EDTA) (Japanese published application 54-156 139), polymethacrylic acid and polyvinyl alcohol (German published application 2 755 319), cross-linked polyhydroxy ethyl methacrylate combined with other polymers (German published application 2 755 256) or even polyethylene oxide (U.S. Pat. No. 3 518 124) have been proposed as suitable additives. This list by no means claims to be exhaustive; however, it must be stated that none of these proposals has found acceptance because the effect was either too small or not maintained for a sufficiently long time.
Other proposals for delaying the antimony poisoning with the aid of separators are based on physical interactions: Japanese published application 50-12 537 attributes the effect to low average pore diameter, although there are many examples demonstrating the opposite. German published application 32 22 361 claims an enlargement of the inner surface of the separator by coating or detaching soluble constituents, to increase the adsorption capacity for antimony ions. The thus achievable delay in the antimony poisoning can, however, not justify the costly production procedure.